bcachefs: If we run merges at a lower watermark, they must be nonblocking

Fix another deadlock related to the merge path; previously, we switched
to always running merges at a lower watermark (because they are
noncritical); but when we run at a lower watermark we also need to run
nonblocking or we've introduced a new deadlock.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Reported-and-tested-by:  <s@m-h.ug>